The present invention relates to dynamometers and in particular to electromechanical devices for simulating road load and vehicle inertia forces for testing vehicles in place.
Vehicle dynamometers are used primarily for two purposes; as measuring devices for determining the torque and/or horsepower output of a vehicle, and as simulation devices for simulating the inertia and road load forces to which a vehicle is normally subjected during actual operation of the vehicle. The present invention is concerned principally with the latter application.
Dynamometer systems, when used as simulators, typically comprise a mechanical device, such as a flywheel, for simulating the inertia for a vehicle, a power absorption unit (PAU) for simulating road load forces, and a system controller for controlling the force output of the PAU. The inertia of a vehicle is a function of the vehicle's weight and is the force which must be overcome for the vehicle to accelerate or decelerate. Road load forces on the other hand are those forces which must be overcome to maintain vehicle speed and include such factors as breakaway torque, rolling friction and windage.
The classic formula for calcuating the force output of a vehicle is: EQU F=(A+Bv+Cv.sup.2)+I(dv/dt)
where:
v=velocity of the vehicle PA1 I=vehicle weight and moment of inertia of its rotating parts PA1 dv/dt=acceleration of the vehicle PA1 A=load coefficient due to static friction; e.g. breakaway torque PA1 B=load coefficient due to kinetic friction; e.g. rolling friction PA1 C=load coefficient of the velocity squared; e.g. windage PA1 (1) PAU response time PA1 (2) ratio of electric to mechanical inertia PA1 (3) response time of accelerometer PA1 (4) value of acceleration PA1 (5) overall torque transducer and system controller response time, PA1 F.sub.t =the force reading from the torque transducer, PA1 V=the measured roll surface speed, and PA1 I.sub.r =the mechanical inertia "outside" the torque transducer loop; (i.e., the inertia of the mechanical components between the driven wheels of the vehicle and the torque transducer.)
More simply, the above force equation can be expressed as: EQU F=RL+I(dv/dt)
where "RL" stands for "road load" forces and is equivalent to the expression "A+Bv+Cv.sup.2."
Many conventional dynamometer systems simulate inertia force (I dv/dt) utilizing a combination of declutchable flywheels. These dynamometers simulate road load forces with a power absorption unit which typically comprises either a DC motor, an eddy current brake, or a hydrokinetic brake. Although flywheels provide good simulation of inertia forces in certain applications, substantial disadvantages are posed by their use in applications where flexibility, low cost, and accuracy of simulation are required. For example, the number of different inertia values that can be simulated by a given set of flywheels is limited. Moreover, a larger selection of inertia values can only be realized by increasing the number of flywheels, which in turn increases the cost and mechanical complexity of the system while decreasing system reliability due to the correspondingly large number of clutches required. In addition, flywheels add parassitic friction and windage losses to the dynamometer system which introduce an error factor into the road load simulation. Moreover, this error factor is difficult to compensate for because it varies each time a new flywheel combination is selected.
The above-noted problems with flywheels are substantially avoided by providing a dynamometer system that utilizes the power absorption unit to simulate both road load and inertia forces. Typically, this latter type of dynamometer system still uses one flywheel, but requires no mechanical clutches.
Most conventional dynamometer systems that use the PAU for both road load and inertia simulation utilize the F=RL+I dv/dt formula to calculate the force output to be simulated by the power absorption unit (PAU). Specifically, the appropriate constants for A, B, C and I are entered into the system controller and the velocity and acceleration values calculated from the information received from a speed sensor connected to the rollers being driven by the driving wheels of the vehicle.
The principal disadvantage with this approach is the inherent lag time of the system in responding to changes in acceleration. In particular, this method determines the inertia force to be simulated by first measuring vehicle acceleration, multiplying that acceleration value(s) by the inertia weight to be simulated, and then simulating the product as the inertia force. While this procedure produces accurate results during states of constant speed or acceleration, it fails to yield accurate inertia simulation while acceleration is changing. This inaccuracy can be explained when it is considered that: (1) acceleration is the result of applying force to a mass (inertia), and (2) force and inertia are the effect that precede acceleration. Therefore, a correct acceleration can only occur after the proper force and mass (inertia) have been established (not before). For this reason it is desirable to eliminate the use of acceleration as an input for calculating inertia force since the proper acceleration does not occur until the correct inertia force has been simulated. Rather, it is preferable that acceleration be treated as an output with force and inertia mass used as inputs. Of course, during constant accelerations it no longer matters if the above sequence is reversed since the acceleration that follows any given point in time is equal to the value that preceded it. However, until a steady state condition is achieved, a classical dynamometer will go into a state of search. In other words, for that brief period of time immediately following a change in acceleration the measured acceleration term in the above formula will be incorrect until the system "catches up". In actuality, the system corrects for changes in acceleration by performing a series of successive approximations until the appropriate PAU force is obtained. The length of this lag time varies from system to system, depending on the following conditions:
and can be as great as several seconds. This matter is typically complicated further by the additional lag time required to measure acceleration. Hence, the amount of time required to accumulate sufficient new data to calculate the appropriate new PAU force value is further increased.
To improve the response time of conventional dynamometer systems, another approach has been proposed which is principally distinguishable from the classic approach discussed above in that it measures the torque output of the vehicle--a more rapidly changing quantity--and from that calculates the velocity at which the PAU should be operating. This is accomplished simply by solving the force equation provided above for velocity which gives the following: ##EQU1## where F.sub.t is now the torque output of the vehicle as measured by a torque transducer. Although this approach succeeds in reducing the lag time associated with the original method discussed, it has two principal disadvantages. First, the torque transducer must be located as closely as possible to the driving wheels of the vehicle so that the torque signal produced by the transducer is truly representative of the force output of the vehicle. This typically entails placement of the torque transducer between the rollers and the flywheel, which is acceptable as long as the inertia of the rollers does not represent a significant percentage of the total inertia of the system. Secondly, the control quantity used for the PAU is speed rather than force. Since the PAU, which typically comprises an SCR-driven direct-current motor, is in reality a torque machine, the output of the PAU can best be controlled with a minimum of delay time by providing it with a torque input signal. A similar analogy can be drawn with an electrical heater. The proper temperature can be reached more rapidly by determining directly the amount of current to be supplied to the heater to produce the desired temperature output rather than by simply "telling" the heater the temperature level desired.
The present invention proposes a third method of dynamometer simulation control which utilizes both a measured force reading and a speed reading to calculate directly the desired force output of the power absorption unit (PAU). More particularly, the control system of the present invention recognizes that the force reading from a torque transducer not positioned directly at the true force output of the vehicle. The present system accordingly utilizes the following equation to determine the actual force output of the vehicle: EQU F=F.sub.t +I.sub.r (dv/dt)
where:
As a consequence, since the force outside the torque loop is being accounted for, the torque transducer is free to be placed between the flywheel and the d.c. motor (PAU) where the torque loads are less severe, thus permitting the use of a smaller more accurate transducer. The present system also permits the torque transducer to be placed next to the rollers or even directly on the vehicle itself without deterioration of simulation accuracy. It should be noted that none of the prior art systems previously described allow this flexibility.
From the above force equation, the present control system proceeds to calculate what the acceleration of the vehicle should be, given its total force output and its current speed, and then calculates the corresponding force output that should be assigned to the PAU to produce the theoretical acceleration. Simultaneously, the inertia within the torque loop is calculated and compared with the assigned loop inertia value to produce an error signal that is added to the designated PAU force value to correct any deviations.
Thus, it will be seen that the dynamometer control system of the present invention virtually eliminates the lag time problems associated with most prior art dynamometer control systems by providing a novel method of feedback control which produces an exact calculated value for the force output of the PAU. Moreover, the force output of the PAU in the present system is controlled directly rather than indirectly through control of its speed.
In addition, to further improve the precision of the system, the present invention includes a novel speed conditioning circuit which derives a highly accurate velocity signal from the output of the speed sensor without sacrificing system response time. More particularly, the speed conditioning circuit (1) counts the speed pulses received from the speed sensor within a predetermined minimum time period plus the first pulse received thereafter, (2) measures the exact time elapsed from the first pulse to the last pulse, and (3) divides the count total by the total elapsed time period. In this manner, the measured time period is not fixed at some arbitrary value but is varied in accordance with the rate at which speed pulses are received. Hence, it will be seen that the percentage error in the velocity signal is substantially reduced without significantly extending the length of the measured time period.
Additional objects and advantages of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :